Today, most information is stored and transmitted digitally. Thus, audio files being played back by a user are generally digitally-encoded audio information. At times, a device may be requested to play back two sounds simultaneously. Mixing of two audio signals as two analog signals is generally not difficult. Audio sources are generally converted from digital to analog signals and then the one or more audio signals is played back through a speaker. A key component of a circuit for reproducing audio information as sounds for a user is thus a digital-to-analog converter (DAC). Conventionally, audio information is converted to common analog signals regardless of the stored format of the audio information. The converted analog signals of multiple sources may then be mixed together, in analog processing, to obtain a speaker output from multiple sources.
Although computer circuitry can be either digital processing circuitry or analog processing circuitry, digital processing circuitry is often cheaper to design and manufacture than analog processing circuitry. Further, digital processing circuitry may consume less space and consume less power than equivalent analog processing circuitry. Thus, digital processing circuitry may be preferred over analog processing circuitry for certain devices, such as mobile devices. In these devices, processing of the audio information is generally performed in the digital domain, rather than the analog domain. In particular, the processing is performed prior to or as part of conversion of audio information to analog signals by the digital-to-analog converter (DAC).
Audio information of different digital formats is difficult to combine together for simultaneous output at a speaker using a single output path to the speaker through the digital-to-analog converter. One challenge is that the audio information may be stored as different resolution and different quality audio. For example, a high resolution audio digital-to-analog converter (DAC) that is designed for the portable and handset space is faced with an interesting problem. While the handset is playing back high-resolution audio streams, there may arise a need to use the audio playback channel to alert the consumer to incoming emails, text messages, or other notifications. There is a sizable library of short audio files that have been created for various types of notification, which are typically stored as WAV, AIFF, MP3, or other PCM-based file formats. The sample rates and bit depths of the PCM-based files are different from high fidelity (“Hi-Fi”) audio files, which increases the difficulty of generating a single audio output from different audio streams.
One conventional system for generation of an audio output from multiple sources is shown in FIG. 1. FIG. 1 is a block diagram illustrating an example of mixing of multiple PCM source files according to the prior art. A first PCM source 102, such as music, may be played back through an audio mixer 106 to generate audible sounds from a speaker 108. A second PCM source 104 may also be supplied to the audio mixer 106, which combines and scales signals from the sources 102 and 104. Because of the common digital format of the sources 102 and 104, the audio mixer 106 can combine information from the two sources in the digital domain with relatively little processing prior to conversion to an analog signal for output to speaker 108. However, recent interest in high-definition, or high-fidelity audio, which is generally stored in a different digital format than existing PCM files, has necessitated a change in operation of audio mixer 106 to support these Hi-Fi formats. Although conventional audio mixers are capable of processing these Hi-Fi formats, they do not support the mixing of dissimilarly formatted audio information, such as when one audio source is a pulse code modulation (PCM) source and one audio source is a pulse density modulation (PDM) source.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved electrical components, particularly for audio processing employed in consumer-level devices, such as mobile phones. Embodiments described herein address certain shortcomings but not necessarily each and every one described here or known in the art.